1. Field Of The Invention
The present invention generally relates to a die positioning system for a tube bending machine, and more particularly to a die positioning system which moves the die into position at a low force prior to applying relatively high clamping force and automatically retracts the die if an obstacle is encountered.
2. Description of Related Art
Tube bending machines are well-known in the art. In one common type of machine, a tube is secured between a bend die and a clamp die which rotate together, drawing the lead portion of the tube therewith to bend it around the bend die. A pressure die engages an outside wall of the trailing portion of the tube to counter the reaction force of the tube during bending.
The purpose of the clamp die is to clamp the tube on a straight section of the bend die and rotate along with the bend die during the bending operation. The clamp die is typically mounted on a clamp die holder which is either mounted on a moving slide which moves in and out to close and open the tools, or mounted directly on a linkage mechanism which drops away into an arm cavity when in an open position to clear the tube after bending. The clamp die is normally positioned with a hydraulic cylinder which operates at a single pressure and a limit switch is typically used to indicate when the clamp die is fully closed. Typically, after the first bend is completed on a CNC tube bending machine using a carriage feed for the tube, the tools are opened before the bend arm can be returned to its home position for the next bend in the same tube. The arm, therefore, cannot be returned to its home position unless the tube clears the straight portion of the bend area. To do this the clamp die and holder must be out of the way before the carriage can move forward. In a non-drop-away clamp holder, either a double spindle arm is provided, where the inner spindle holds the bend die, so that the clamp die can be returned before the carriage moves forward, or the carriage and arm must be alternately be moved a distance equal to the slide opening distance until the arm returns without any interference with the tube. Another approach has been to use a drop-away clamp die holder mounted on the slide or on a toggle mechanism, which drops the clamp die below the centerline height of the bend die.
Each approach has a number of disadvantages. A double spindle weakens the arms, it still requires a toggle type slide which requires manual set-up for position of the tools, and pressure cannot be varied because of the toggle. The drop-away design mounted on the slide or on a drop-away linkage has the same problems. A further problem with all known designs is that the operator can be hurt if his or her hand is caught in a pinch point between the clamp die and the bend die.